1. Field of the Invention
This invention pertains generally to electromagnetic coils and more particularly to the fabrication of silicone-potted ignition coils and high temperature ignition coils.
2. Description of the Background Art
Conventionally, composite iron powders are coated with various polymeric binders, which insulate a plurality of iron powder particles from one another after the iron powder particles are pressed to form a part. An insulated core formed from iron powder particles may be utilized for AC electromagnetic applications because the magnetic characteristics are similar to a laminated sheet stack core. The binders may be thermoplastic polymeric materials, thermoset polymeric materials, inorganic materials, or a combination of all types of these materials. If a thermoset resin is used, the molded part can be cured (i.e., cross-linked) in an attempt to impart better molding properties. However, upon curing, undesirable results such as the formation of a hard insoluble film on the part's surface may occur. This unwanted film can cause inhibition problems with the potting materials, dimensional control problems, and performance difficulties. The film formation results in added expense for its removal, imposes additional processing time, and may degrade the dimensional control causing the need for machining the part.
Although various organic and inorganic materials have been used to coat the surface of a powdered metal, more frequently organic binders such as thermoplastics and thermosets are used. Thermoset resins are reactive materials, which cross-link or react upon exposure to various sources of energy such as heat, ultraviolet radiation, and pressure. When a powdered metal is coated with a thermoset binder, the binder is typically left unreacted (i.e. uncured) until the part is molded. Upon subsequent cross-linking, reaction by-products can be detrimental to the core or to the remainder of the system. These reaction by-products can include evolved gases, moisture, or a physical interaction with other organic materials (e.g. lubricants). If the thermoset coating on powdered metal (PM) is cross-linked when making static physical contact (i.e. at a stage prior to molding while still a free flowing powder), the powdered metal agglomerates or worse forms a large solid mass, taking the shape of its container.